This invention relates to a circuit for actuating a display having liquid crystal segments or luminescent diodes in accordance with an analogue input voltage.
In an automobile, a display can be often found which has plural display segments closed of liquid crystal composition or luminescent diodes. For example, a speedometer which employs such display has plural display segments disposed in a straight line or semicircular form on a dashboard of the car. A circuit for actuating such display has a comparing circuit and an actuating circuit. The comparing circuit is provided for determining a number of the display segments to be energized and activated according to an analogue input voltage from a detector, such as a tachogenerator. The actuating circuit is provided for energizing and activating the display segments in response to the output signals form the comparing circuit. According to the prior art, the comparing circuit is constructed with plural unipolar transistors and a resistance which includes plural resistors connected in series. The analogue input voltage is applied to one terminal of the resistance, the other terminal of which is grounded. Each of the junctions of the resistors connected in series is applied to the base of corresponding transistors. Each transistor is turned in when the voltage applied to the base thereof exceeds a threshold value. Each transistor of the comparing circuit provides a output signal at a collector thereof. The actuating circuit energizes and activates the display segments in response to the output signals from the comparing circuit. Accordingly, the display segments of the number in accordance with the analogue input voltage are energized and activated.
Such a circuit, however, has drawbacks as follows. In the case of employing a relatively large number of the display segments, for example when ten display segments are installed, the circuit tends to make an error particularly in small range of the analogue input voltage. That is due to the fact that the threshold voltage of the transistor is relatively small; e.g. on the order of 0.6 volt in case of a silicon N-P-N transistor. In the small range of the analogue input voltage, for example nearly equal to the threshold value, voltages at the junctions of the resistors connected in series come very close to one another. Therefore, a transistor or transistors which should not be turned on are turned on erroneously. Further, the comparing circuit which is constructed with such plural bipolar transistors tends to consume a relatively large amount of electric power. The unipolar transistor also has a drawback that it requires the relatively large area on a semiconductor chip of an integrated circuit. Such comparing circuit is, therefore, not suitable for fabrication in the form of the integrated circuit.